1. Field of the Invention
The present invention relates to a method and apparatus for polishing paper. In particular, the method and apparatus utilize a polishing belt and cylinder in order to obtain the desirable properties that mechanical shear action imparts to paper. The present invention offers an optimal way to apply mechanical shear energy to paper without some of the drawbacks found with earlier devices.
2. Description of the Background Art
To impart mechanical energy to a paper surface, a frictional device is left with only three variables; contact force, sliding distance and the coefficient of friction of the frictional material. This relationship can be expressed as E.sub.m =Nud where E.sub.m =Mechanical energy, N=Normal or contact force, u=coefficient of friction of the polishing surface and d=sliding distance.
The devices considered to be the prior art are at either end of the spectrum, with respect to contact force and distance. These prior art devices have relatively low coefficients of friction for the frictional surface in most cases.
One prior art example uses brush polishing devices which rely on low contact forces and large slip distances. Surface speed differentials typically approach 15,000 feet per minute (fpm). Brush polishers are limited to material operating speeds of 1,500 fpm by mechanical limitations of the rapidly spinning brushes and bristle performance.
One prior art device disclosed in U.S. Pat. No. 4,089,738 to Kankaanpaa also describes stationary devices which drag hard surfaces against paper with higher contact forces compared to brush polishers. The slipping distances between differentials used in these relatively stationary devices are essentially directly related to web speed. Stationary frictional devices as described by Kankaanpaa generate tremendous heat and wear requiring the frictional surfaces to be made of steel or ceramic with relatively low coefficients of friction. The potential for scratching or the chance for developing a machine direction character on the paper is high because there is little opportunity to clean the device during operation, the polishing surface is very hard and the speed differentials are still relatively high (2,500 to 5,000 fpm).
An invention described in U.S. Pat. No. 2,349,704 to Clark uses soft rollers with a polishing powder to develop a high coefficient of friction between the paper and the frictional device. This relatively high coefficient of friction device along with relatively high speed differentials results in micro-scratching with presumably low contact forces.